The present invention refers to a heat fixing unit for an electrophotographic copying machine of the type in which a copy-sheet, having an unfixed toner image on one side thereof, is advanced between two cooperatively rotated rolls while pressure is applied therebetween and, at least the roll in contact with the toned side of the sheet, is heated to a temperature suitable to fix toner to the sheet.
A drawback of the fixing units of the above type is due to sticking of toner particles, carried out from the sheet, to the surface of the rolls; which particles may successively adhere to a following copy-sheet advanced between the rolls so forming thereon undesired toned images or tracks.
This drawback is known as "offset phenomena" and can be obviated by coating the surface of the rolls with suitable antisticky, heat resistant parting material such as tetrafluoroethylene, sold under the trademark Teflon, or silicone rubber, and by wetting the coated surface of the rolls with antisticky or parting liquid, such as silicone oil.
Fixing units are known in which the silicone oil is continuously applied to the fusing rolls by a dispensing roll rotated in contact with a fusing roll and partially immersed in a silicone oil tank.
This last type of toner antisticky liquid supply device has the drawback of causing an unnecessary high consumption of silicone oil and also, due to the continuous wetting, with a consequent increase in the maintenance cost of the machine, a lowering of the adhesion of the antisticky coating to the metal core of the roll.